Instrument for ascertaining Refracting Indices, fyc. 511 



while the height of the shorter bar b retains the original length 

 proposed by Dr. Wollaston of ten inches, and the indicator c, 

 as a consequence, has only the half of this length, that is to 

 say five inches. The bottom bar d is about two feet in length, 

 and has a dove-tailed groove or furrow ploughed in it through- 

 out its whole length ; in this groove a piece e is made to slide 

 easily, which may be clamped or made fast by means of a 

 thumb-screw in any part that may be required. To this sliding 

 piece is attached a hinge, and to this hinge is also attached 

 one of the sliding bars 1, the other sliding bar 2 being hinged 

 to the bar b. These sliding bars are capable of being fixed 

 at any required length, by means of a clamping-screw. The 

 bar b is hinged to an immoveable blocky of about two inches 

 square, having an excavation of about three quarters of an inch 

 square formed in it, for the purpose of preventing the sub- 

 stances submitted to experiment, on being placed on the base 

 of any one of the prisms, from coming into contact with the 

 wood of which the block is made. Exactly in the middle of 

 the bar b is hinged the indicator c, which is of brass and is filed 

 to a very sharp edge ; it is precisely five inches in length from 

 the centre of the hinge to its extremity. This indicator may 

 be slid along the graduated scale g which is laid down on the 

 upper surface of the bottom bar d, and by means of the pres- 

 sure of the short bar b to which it is attached, will remain in 

 close contact with the graduated scale in whatever position 

 the bars are capable of being placed ; the sharp edge of the 

 indicator is always perpendicular (provided the lengths have 

 been duly attended to, by which is meant the precise distances 

 of the centres of motion of the hinges from each other) to the 

 axis of the hinge which connects the bar b with the moveable 

 bar a. 



It now only remains to be stated in what manner the instru- 

 ment is to be adjusted, which is to be effected in the follow- 

 ing manner: — When a piece of glass has been selected that 

 is capable of giving with the substance under examination a 

 total reflexion at a mean incidence (by which is meant, when 

 the total reflexion occurs at an angle varying from about 35° 

 to 65°), supposing this to be the case with a prism whose re- 

 fractive power is 1*635, such as would be required to obtain 

 the refractive power of any or most of the fixed oils, then the 

 adjustable bar is to be made sixteen inches and thirty-five 

 hundredths of an inch in length between the centres of the 

 pins of the hinges, and the bottom bar to be shifted until the 

 brass edge of the indicator stands at *635 on the scale ; the 

 substance on being applied to the base of the prism is then 

 to be put into its situation on the block, and the whole ap- 



